1. Field of the Invention
The present invention relates generally to the field of telemetry systems, and more particularly to synchronization methods within telemetry systems.
2. Description of the Prior Art
In a medical telemetry system an implantable device, such as a pacemaker, is monitored regularly by an external programming device. A physician treating the patient can thereby obtain important information, for example in order to evaluate the heart activity of the patient. The physician is also able to make changes to the settings of the implantable device if such need arises. In order to accomplish this bidirectional communication, a radio connection has to be set up between the programming device, also denoted base station (BS) or wand, of the telemetry system and the implantable device.
The communication has to fulfil different requirements, for example as stipulated by Federal Communications Commission (FCC) in the USA and by European Telecommunications Standards Institute (ETSI) in Europe. The available spectrum has to be used as efficiently as possible and the standards specify some requirements that have to be fulfilled. These requirements have to be taken into consideration when designing a MICS (Medical Implant Communication Service) system and the devices used within the system.
In particular, for medical telemetry systems, FCC MICS and ETSI ULP-AMI standards have allocated frequencies from 402 MHz to 405 MHz to be used in ten channels that are 300 kHz wide and shared among other users. Only one channel is allowed for use in a dialog from a base station to an implantable medical device (IMD), and therefore channels cannot be combined to achieve a broader channel. Further, separate channels for uplink (IMD to BS) and downlink (BS to IMD) use are not allowed. Another requirement is that the transmitter power should not exceed 25 μW (equivalent to −16 dB) in a channel. The MICS channel spacing and transmit power limit are illustrated in FIG. 1.
The standards define the radiated power as equivalent isotropic radiated power, eirp, which takes into account the properties of the radiating source. For an implanted device the radiated power of 25 μW is measured at the body surface, not at the device antenna itself. In fact, the standards postulate that the measurement is to be done with the device soaked in a specified torso phantom, filled with a fluid with electromagnetic properties that resembles the human body.
With reference to FIGS. 2a and 2b, the channel width is defined at the lower and upper frequencies, flow and fhigh, respectively, at which the signal level is 20 dB below the maximum level of the modulated carrier. The type of modulation used in the RF telemetry transmitter benefits from this definition and uses the frequency spectrum rather extensively, but still within the limitations of the standard. The slowest communication speed uses a 2-level frequency shift keying modulation spectrum mask, even if its nearest sidelobes intrude on the adjacent channel.
A radio receiver with perfect selectivity rejects the entire signal that is not in the channel that the receiver is tuned to. However, in practice all radio receivers hear a signal from the adjacent channels. This is known as crosstalk or co-channel interference.
When a digital receiver is tuned to one channel, all data transmitted on other channels are undesired data. Unfortunately, this data can accidentally be received anyway because of the above-described crosstalk. The crosstalk behaviour may cause establishment of a communication session on different channels for two communication peers, i.e. on different channels for a BS and an IMD. This reduces the ability to read parameters and to perform fast and accurate programming. Further, having several communication sessions in an area increases the probability for crosstalk. Further yet, this is not allowed in accordance with the earlier mentioned standards.
Crosstalk may be controlled by various radio resource management schemes. Crosstalk can, for example, be combated by controlling parameters such as transmit power, channel allocation, modulation scheme, error coding scheme etc. All these methods require extensive planning resources and are performed on a system level with the objective to utilize the limited radio spectrum resources and radio network infrastructure as efficiently as possible. Although reducing crosstalk, such resource management schemes cannot entirely eliminate the crosstalk.
It would thus be desirable to provide improved means for reducing or even eliminating crosstalk between a base station and a telemetry receiver device of a telemetry system. Further, it would be desirable to provide such method, fulfilling the requirements set out by the above-mentioned standards.